Plant proteins and enzymes have long been exploited for many purposes, from viable food sources to biocatalytic reagents, or therapeutic agents. During the past decade, the development of transgenic and transfected plants and improvement in genetic analysis have brought renewed scientific significance and economical incentives to these applications. The concepts of molecular plant breeding and molecular plant farming, wherein a plant system is used as a bioreactor to produce recombinant bioactive materials, have received great attention.
Many examples in the literature have demonstrated the utilization of plants or cultured plant cells to produce active mammalian proteins, enzymes, vaccines, antibodies, peptides, and other bioactive species. Ma et al. (Science 268:716-719 (1995)) were the first to describe the production of a functional secretory immunoglobulin in transgenic tobacco. Genes encoding the heavy and light chains of murine antibody, a murine joining chain, and a rabbit secretory component were introduced into separate transgenic plants. Through cross-pollination, plants were obtained to co-express all components and produce a functionally active secretory antibody. In another study, a method for producing antiviral vaccines by expressing a viral protein in transgenic plants was described (Mason et al., Proc. Natl. Acad. Sci. USA 93: 5335-5340 (1996)). The capsid protein of Norwalk virus, a virus causing epidemic acute gastroenteritis in humans was shown to self-assemble into virus-like particles when expressed in transgenic tobacco and potato. Both purified virus-like particles and transgenic potato tubers when fed to mice stimulated the production of antibodies against the Norwalk virus capsid protein. Alternatively, the production and purification of a vaccine may be facilitated by engineering a plant virus that carries a mammalian pathogen epitope. By using a plant virus, the accidental shedding of virulent virus with the vaccine is abolished, and the same plant virus may be used to vaccinate several hosts. For example, malarial epitopes have been presented on the surface of recombinant tobacco mosiac virus (TMV) (Turpen et al. BioTechnology 13:53-57 (1995)). Selected B-cell epitopes were either inserted into the surface loop region of the TMV coat protein or fused into the C terminus. Tobacco plants after infection contain high titers of the recombinant virus, which may be developed as vaccine subunits and readily scaled up. In another study aimed at improving the nutritional status of pasture legumes, a sulfur-rich seed albumin from sunflower was expressed in the leaves of transgenic subterranean clover (Khan et al Transgenic Res. 5:178-185 (1996)). By targeting the recombinant protein to the endoplasmic reticulum of the transgenic plant leaf cells, an accumulation of transgenic sunflower seed albumin up to 1.3% of the total extractable protein could be achieved.
Work has also been conducted in the area of developing suitable vectors for expressing foreign genetic material in plant hosts. Ahlquist, U.S. Pat. No. 4,885,248 and U.S. Pat. No. 5,173,410 describe preliminary work done in devising transfer vectors which might be useful in transferring foreign genetic material into plant host cells for the purpose of expression therein. Additional aspects of hybrid RNA viruses and RNA transformation vectors are described by Ahlquist et al in U.S. Pat. Nos. 5,466,788, 5,602,242, 5,627,060 and 5,500,360 all of which are herein incorporated by reference. Donson et al, U.S. Pat. No. 5,316,931 and U.S. Pat. No. 5,589,367, herein incorporated by reference, demonstrate for the first time plant viral vectors suitable for the systemic expression of foreign genetic material in plants. Donson et al. describe plant viral vectors having heterologous subgenomic promoters for the systemic expression of foreign genes. The availability of such recombinant plant viral vectors makes it feasible to produce proteins and peptides of interest recombinantly in plant hosts.
Elaborate methods of plant genetics are being developed at a rapid rate and hold the promise of allowing the transformation of virtually every plant species and the expression of a large variety of genes. However, in order for plant-based molecular breeding and farming to gain widespread acceptance in commercial areas, it is necessary to develop a cost-effective and large-scale purification system for the bioactive species produced in the plants, either proteins or peptides, especially recombinant proteins or peptides, or virus particles, especially genetically engineered viruses.
Some processes for isolating proteins, peptides and viruses from plants have been described in the literature (Johal, U.S. Pat. No. 4,400,471, Johal, U.S. Pat. No. 4,334,024, Wildman et al., U.S. Pat. No. 4,268,632, Wildman et al., U.S. Pat. No. 4,289,147, Wildman et al., U.S. Pat. No. 4,347,324, Hollo et al., U.S. Pat. No. 3,637,396, Koch, U.S. Pat. No. 4,233,210, and Koch, U.S. Pat. No. 4,250,197, the disclosure of which are herein incorporated by reference). The succulent leaves of plants, such as tobacco, spinach, soybean, and alfalfa, are typically composed of 10-20% solids, the remaining fraction being water. The solid portion is composed of a water soluble and a water insoluble portion, the latter being predominantly composed of the fibrous structural material of the leaf. The water soluble portion includes compounds of relatively low molecular weight (MW), such as sugars, vitamins, alkaloids, flavors, amino acids, and other compounds of relatively high MW, such as native and recombinant proteins.
Proteins in the soluble portion of plant biomass can be further divided into two fractions. One fraction comprises predominantly a photosynthetic protein, ribulose 1,5-diphosphate carboxylase (or RuBisCO), whose subunit molecular weight is about 550 kD. This fraction is commonly referred to as "Fraction 1 protein." RuBisCO is abundant, comprising up to 25% of the total protein content of a leaf and up to 10% of the solid matter of a leaf. The other fraction contains a mixture of proteins and peptides whose subunit molecular weights typically range from about 3 kD to 100 kD and other compounds including sugars, vitamins, alkaloids, flavors, amino acids. This fraction is collectively referred to as "Fraction 2 proteins." Fraction 2 proteins can be native host materials or recombinant materials including proteins and peptides produced via transfection or transgenic transformation. Transfected plants may also contain virus particles having a molecular size greater than 1,000 kD.
The basic process for isolating plant proteins generally begins with disintegrating leaf biomass and pressing the resulting pulp to produce "green juice". The process is typically performed in the presence of a reducing agent or antioxidant to suppress unwanted oxidation. The green juice, which contains various protein components and finely particulate green pigmented material, is pH adjusted and heated. The typical pH range for the green juice after adjustment is between 5.3 and 6.0. This range has been optimized for the isolation of Fraction 1 protein (or ribulose 1,5-diphosphate carboxylase). Heating, which causes the coagulation of green pigmented material, is typically controlled near 50.degree. C. The coagulated green pigmented material can then be removed by moderate centrifugation to yield "brown juice." The brown juice is subsequently cooled and stored at a temperature at or below room temperature. After an extended period of time, e.g. 24 hours, ribulose 1,5-diphosphate carboxylase is crystallized from the brown juice. The crystallized Fraction 1 protein can subsequently be separated from the liquid by centrifugation. Fraction 2 proteins remain in the liquid, and they can be purified upon further acidification to a pH near 4.5. Alternatively, the crystal formation of ribulose 1,5-diphosphate carboxylase from brown juice can be effected by adding sufficient quantities of polyethylene glycol (PEG) in lieu of cooling.
The basic process for isolating virus particles is described in Gooding et al. (Phytopathological Notes 57:1285 (1967), the teaching of which are herein incorporated by reference). To purify Tobacco Mosaic Virus (TMV) from plant sources in large quantities, infected leaves are homogenized and n-butanol is then added. The mixture is then centrifuged, and the virus is retained in the supernatant. Polyethylene glycol (PEG) is then added to the supernatant followed by centrifugation. The virus can be recovered from the resultant PEG pellet. The virus can be further purified by another cycle of resuspension, centrifugation and PEG-precipitation.
Existing protocols for isolating and purifying plant viruses and soluble proteins and peptides, however, present many problems. First, protein isolation from plant sources have been designed in large part for the recovery of Fraction 1 protein, not for other biologically active soluble protein components. The prior processes for large-scale extraction of F1 proteins was for production of protein as an additive to animal feed or other nutritional substances. Acid-precipitation to obtain Fraction 2 proteins in the prior art is not effective, since most proteins denature in the pellet form. This is especially troublesome for isolating proteins and peptides produced by recombinant nucleic acid technology, as they may be more sensitive to being denatured upon acid-precipitation. Second, the existing methods of separation rely upon the use of solvents, such as n-butanol, chloroform, or carbon tetrachloride to eliminate chloroplast membrane fragments, pigments and other host related materials. Although useful and effective for small-scale virus purification, using solvents in a large-scale purification is problematic. Such problems as solvent disposal, special equipment designs compatible with flammable liquids, facility venting, and worker exposure protection and monitoring are frequently encountered. There are non-solvent based, small-scale virus purification methods, but these are not practical for large scale commercial operations due to equipment and processing limitations and final product purity (Brakke Adv. Virus Res. 7:193-224 (1960) and Brakke et al. Virology 39: 516-533(1969)). Finally, the existing protocols do not allow a streamline operation such that the isolation and purification of different viruses, proteins and peptides can be achieved with minimum modification of a general purification procedure.
There is a need in the art for an efficient, non-denaturing and solvent-limited large-scale method for virus and soluble protein isolation and purification. This need is especially apparent in cases where proteins and peptides produced recombinantly in plant hosts are to be isolated. The properties of these proteins and peptides are frequently different from those of the native plant proteins. Prior art protocols are not suitable to isolate recombinant proteins and peptides of interest. In addition, the vast diversity of recombinant proteins and peptides from plants and the stringent purity requirement for these proteins and peptides in industrial and medical application requires an efficient and economical procedure for isolating and purifying them. Efficient virus isolation is also of great importance because of the utility of viruses as transfection vectors and vaccines. In some situations, proteins and peptides of interest may be attached to a virus or integrated with native viral proteins (fusion protein), such that isolating the protein or peptide of interest may in fact comprise isolating the virus itself.